Cystic fibrosis (CF) is caused by mutation in a single gene that codes for a 1480 amino acid protein called the cystic fibrosis transmembrane conductance regulator (CFTR). The mutated CFTR results in a marked decrease in Cl- permeability in several cell types, an increase in Na+ transport in airway cells, and differences in mucus composition. The best present evidence indicates that CFTR is a 6-9 pS (at 22-37 degrees C respectively), linear, cAMP-regulated Cl- channel that requires PKA- dependent phosphorylation and ATP hydrolysis to function. The possibility that CFTR has additional functions remains to be assessed. Most mutations in the CFTR gene that give rise to cystic fibrosis also lead to abnormal processing of CFTR. However, at least when overexpressed in certain non-epithelial cells, the deltaF508 mutant form of CFTR retains partial function. Therapies for CF could attempt to add a normal CF gene to affected cells (gene therapy); add normal CFTR protein (protein therapy), or stabilize mutant CFTR and facilitate its trafficking to the membrane. These methods share the advantage that they could be therapeutically successful regardless of what CFTR normally does. If decreased Cl- conductance is a major factor producing CF airway dysfunction, therapies might also be based upon pharmacological strategies to increase the open probability of any existing mutant CFTR in the membrane, or therapies might seek to activate other Cl- channels in the membrane to bypass the CFTR cl- channel. To help provide a grounding for therapies, we propose experiments in four areas. Studies of CFTR expression will test the hypothesis that levels of CFTR mRNA, protein and cAMP-activated GCl- are directly related. Studies of the single channel kinetics of CFTR-associated channels will determine how channel kinetics are altered by PKA levels, ATP levels, temperature, PKC, and various pharmacological treatments. Experiments with deltaF508 and other mutant forms of CFTR will determine if residual function can be detected in native epithelial tissues, especially with the use of xanthines and related compounds; we also propose to study the single channel kinetics of selected mutant forms of CFTR. Finally, we have identified other species of epithelial Cl- channels and propose experiments to characterize these channels, some of which might serve as potential pathways for bypassing defective CFTR.